The invention also concerns a timepiece provided with such a display assembly wherein the first display device is formed for example by the dial of the timepiece and the second display device is arranged between the crystal and the dial or merged with the crystal. The invention concerns more particularly such a timepiece wherein the dial cooperates with an analogue display.
According to another aspect of the invention, the information given by the first display device is permanently visible and that given by the second display device can be requested on demand to appear in a light colour on a dark background or conversely in a dark colour on a light background.
A display assembly including two superposed display devices able to take two different states wherein one of the display devices is visible to the exclusion of the other, is already known from European Patent No. 0 926 574 in the name of the Applicant. Such a display assembly in its application to a wristwatch is schematically shown in a diametral cross-section in FIG. 1, the operating principle being given by FIGS. 2A and 2B.
The watch shown in FIG. 1A, designated by the general reference 1, includes, in a conventional manner, a middle part 2, a back cover 3 and a crystal 20 together delimiting a case in the bottom part of which is arranged an electronic clockwork movement 4 powered by a battery 5. Movement 4 includes an electronic time-keeping circuit associated with a drive device (not shown), hour, minute and second hands 12, 14 and 16 moving above a dial 18 bearing hour symbols (not shown).
Hands 12, 14 and 16 and dial 18 constitute the first display device, which is of the analogue type in the example illustrated, and designated by the general reference 22.
This wristwatch 1 further includes a second display device of the digital type, designated by the general reference 24 and arranged between dial 18 and crystal 20. With reference again to FIGS. 2A and 2B, it can be seen that second display device 24 is formed of a sandwich type structure including, moving from crystal 20 to dial 18, an absorbent linear polariser 40, a liquid crystal display cell 26, an absorbent linear polariser 42 crossed with polariser 40, a liquid crystal optical valve 28 and a reflective polariser 44 crossed with polariser 42. The switching states of the cell and the valve of the second display device are controlled by a control unit 23 as a function of manipulations effected by means of at least one external control member 9.
Display cell 26 includes, in a conventional manner, a transparent front substrate 30, a back substrate 32 which is also transparent, and a sealing frame 34 forming spacing and closing means delimiting with substrates 30 and 32 a closed cavity containing liquid crystals 27. The opposite faces of substrates 30 and 32 include transparent electrodes respectively 36 and 38 made for example of ITO. In the example illustrated, back electrode 38 extends over the entire surface of substrate 32 and front electrode 36 is configured in segments or digits which can be addressed separately by means of control unit 23, the activated (ON) or non activated (OFF) state being symbolised by the contactor 6 in the diagram of FIG. 2A and in the following diagrams. This cell 26 thus allows alphanumerical characters to be displayed by causing liquid crystals 27 to switch from a transparent state to an absorbent state, or vice versa depending upon the type of liquid crystals used. As will be understood hereinafter, in the application to a wristwatch where energy saving is of great importance, it will be preferable to use liquid crystals which give the cell a transparent state when no voltage is applied and an absorbent state when a voltage is applied.
Optical valve 28, containing liquid crystals 29 and having a contactor 8, has a comparable structure to that of cell 26 and differs from it only in that the two transparent electrodes totally cover the opposite faces of top substrate 31 and bottom substrate 33, so that valve 28 can switch from a totally transparent state to a totally absorbent state, or vice versa depending upon the type of liquid crystals used. For the reason indicated hereinbefore, liquid crystals 29 will preferably be selected to have a transparent state in the absence of voltage.
European Patent No. 0 926 574 only envisages two different operating states for the display assembly, as explained briefly with reference to FIGS. 2A and 2B annexed to the present patent application. Liquid crystals 27 and 29 which are respectively in cell 26 and valve 28 are of the positive anisotropy twisted nematic type.
In a first state shown in FIG. 2A, in which cell 26 and valve 28 are not switched (OFF-OFF state), the natural non polarised light 46 is polarised vertically by first absorbent polariser 40 and designated by the general reference 48. Passing through cell 26, the axis of polarisation undergoes a rotation of 90°, so that the light passes through crossed polariser 42 without being modified. Passing then through valve 28, the axis of polarisation undergoes another rotation of 90° so that the polarised light passes through reflective polariser 44 without being modified to reach ial 18 forming first display device 22.
The light follows the same path during its return travel, so that dial 18 is visible to an observer 50 placed in front of the display assembly.
In a second state shown in FIG. 2B, the segments or digits 36 of the cell 26 are switched (ON state), and valve 28 is also switched (ON state) over the entire surface so that the vertically polarised light emerging from absorbent polariser 40 will have two different types of behaviour. In the zone in which cell 26 is not switched the polarised light follows the same path as previously as far as valve 28 which it passes through without undergoing another rotation so that, since its axis of polarisation is perpendicular to that of reflective polariser 44, it is totally reflected on its return travel concealing dial 18 from observer 50. In the zone of cell 26 in which segments 26 are addressed, the vertically polarised light 48 passes through cell 26 without modification, so that since its axis of polarisation is perpendicular to that of absorbent polariser 42, it is totally absorbed, the addressed segments appearing then in a dark colour on the light background.
According to a second embodiment described in European Patent No. 0926 574, the same visual effect is obtained with liquid crystals of the negative anisotropy twisted nematic type with a construction in which the three polarisers are parallel.
As can be seen, whatever the positive or negative anisotropy of the twisted nematic liquid crystals, the passage from an OFF-OFF switching configuration to an ON-ON switching configuration does not allow an inversion of the contrast to be obtained. In a known manner, such a contrast inversion could be obtained by means of inverse addressing, but at the cost of a demand for energy nine times greater than that which is necessary to address only {fraction (1/10)} of the useful segments for the display, a solution which is unsatisfactory in a system where energy saving is of great importance.
Likewise, according to the teaching of the prior art, one could hope to obtain an inversion of the contrast by rotating a polariser by 90°, which would obviously cause great complication as regards a mechanical or manual drive system, especially for a display assembly of small dimensions, such as that able to be incorporated in a wristwatch.
In order to try to obtain an inversion of the contrast, it also seems natural, to avoid the aforementioned drawbacks, to examine the effect obtained with the device which has just been described by using a third switching possibility, namely the cell in the ON state and the valve in the OFF state, as shown in FIG. 2C. Outside the switching zones of the segments of cell 26, the polarised light passes through valve 28 undergoing another rotation of 90°, so that it is polarised parallel to reflective polariser 44 through which it passes to be returned by the dial through an identical return path to observer. The second display is then shown in dark on the background of the dial, which must then necessarily be a light colour, but does not undergo any contrast inversion.
The second embodiment of European Patent No. 0 926 574 suggests acting on the relative orientation of the three polarisers. The appearances which it would be possible to observe with TN liquid crystals with positive anisotropy, for the two switching configurations able to generate an inversion of the contrast are summarized in the following table recalling those observed in FIGS. 2B and 2C.
AspectDisplay(s) ableaccord-Arrangement ofStateto be observeding topolariserscellvalve& contrastFIG.P40P42P44C26V28of 2nd displayNo.P⊥P⊥PONON2nd D/L [P44]2BONOFF1st et 2nd D/L, if2Cdial lightP//P//PONON1st et 2nd L/D, if3Adial lightONOFF2nd L/D [P42]3BP//P⊥PONON2nd L [P44]/D4A[P42]ONOFF2nd L/D [P42], if4Bdial lightP⊥P//PONON1st et 2nd D/L, if5Adial lightONOFF2nd D/L [P44]5BKey: ⊥ crossed polarisers; // parallel polarisers L/D light colour on dark background; D/L dark colour on light background 
The assembly shown in FIGS. 3A and 3B differs from the aforecited prior art in that the intermediate absorbent polariser 42 was rotated by 90° so that the three polarisers have parallel axes of polarisation. This configuration allows a second display in a light colour on a dark background to be obtained whatever the ON-ON or OFF-OFF switching state, i.e. reversed with respect to the preceding example, but without inversion for the display assembly as such when one passes from one switching state to another. In the assembly shown in FIGS. 4A and 4B, back reflective polariser 44 has also been made to undergo a rotation of 90° and it can be seen that the same visual effect as before is obtained, only with an inversion of the switching configuration. By making intermediate polariser 42 undergo a rotation of 90° as shown in FIGS. 5A and 5B, one returns to the visual aspect of FIGS. 2A and 2B, again without obtaining an inversion of the contrast over the same display assembly.
The object of the present invention is thus to overcome the drawbacks of this prior art by providing a display assembly including two superposed display devices, allowing an inversion of the contrast to be obtained in one of the displays without increasing the energy requirement and without requiring a complex polariser drive mechanism.